| Aliphatic polyester is a kind of environment-friendly material with strong biodegradability.As a binder,it has the advantages of moderate viscosity,fast wetting speed,good adhesion and high transparency.There are some disadvantages in the traditional chemical synthesis process,but the enzyme catalyzed polymerization process is mild,which does not cause adverse reactions or metal residues,and can realize controllable molecular weight and uniform distribution.Pyrotechnic burning is usually fast combustion,insufficient combustion will result in producing pollution of PM and hazardous gas.Pyrotechnic with controllable combustion speed and low temperature and environment-friendly performance are gradually needed.At present,phenolic resin is commonly used as binder for pyrotechnics.The preparation process is not environment-friendly,and a large amount of PM and hazardous gas is produced after the preparation of pyrotechnic.In this paper,polyester was synthesized by biological method as the binder of pyrotechnic and its properties were verified.The mechanism of different binders on the low combustion and environment-friendly performance of pyrotechnic was studied.The specific research results obtained:(1)The synthesis of dibutyl itaconic acid and mono butyl itaconic acid catalyzed by lipase were studied and optimized.Two kinds of itaconate homopolymers were prepared by chemical catalysis homogenization,and the performance was tested.Esterification of itaconic acid and n-butanol was catalyzed by lipase in solvent-free condition.The effects of lipase dosage,reaction temperature,substrate mole ratio and reactor on the conversion of itaconic acid were studied.The conversion of itaconic acid was 61.80%after optimization of reaction conditions.After separation and purification,the purity of dibutyl itaconate and mono butyl itaconate was 97.32%and 96.71%,respectively.Poly[mono butyl itaconate]PMIB and Poly[dibutyl itaconate]PDIB were synthesized by chemical catalytic selfpolymerization of mono butyl itaconate and dibutyl itaconate,respectively.The structure of the polymer was characterized by NMR.The thermal decomposition was detected by thermal analysis,which showed that PDIB was more stable and more thorough decomposition than that of PMIB.(2)The two-step of pre-polymerization and polymerization catalyzed by lipase was used.Two to five kinds of different substrates were polymerized into seven kinds of linear saturated and unsaturated copolyesters with molecular weight of 1000～4000 g/mol.The structure of polyester was analyzed by GPC,FTIR,NMR and acid value detection.The results showed that the biological method was more moderate than chemical polyester,and the molecular weight distribution of polyester products was more uniform.The thermal decomposition characteristics of polyester and phenolic resin were evaluated by thermal analysis.It was found that the residue rate of biological polyester was lower than that of chemical polyester,and polyester was completely decomposed than phenolic resin,and the residue rate of polyester could be reduced by about 88.52%.(3)Different polyesters were used as binder,and based on the zerooxygen balance design formula,the pyrotechnics were prepared by wet mixing method.The performance of safety,mechanical,combustion and environment-friendly of pyrotechnics were tested.The safety performance of pyrotechnic was characterized by friction sensitivity,mechanical sensitivity,moisture absorption and thermal analysis;mechanical properties were characterized by particle hardness and SEM;combustion performance was characterized by closed explosive p-t curve test;environment-friendly performance was characterized by test of PM concentration and gas composition after combustion.The results show that the pyrotechnics with biological polyester binder have good mechanical hardness and safety performance,can reduce the heat emission,slow down the whole combustion process,reduce the particles and hazardous gases produced by combustion.Compared with phenolic resin,53.51%PM,94.53%CO,71.62%NOx and 40.64%residue was decreased during the combustion of pyrotechnic with PDIB binder.(4)The impact mechanism of different binder structure on the slow combustion and environment-friendly performance of pyrotechnic was discussed.The theory of "net pocket" of pyrotechnic binder was found.The three-dimensional structure of the binder and other components of pyrotechnic was proved by FTIR and XPS.The combustion process of pyrotechnic was simulated by programmed heating and open fire ignition.The change of components in combustion process was determined by EDS and XRD.The results showed that the binder formed a "net pocket"structure during the bonding process of pyrotechnic,wrapped the combustible agent and oxidant in each "net pocket" unit to form a number of combustion chambers.The structure and the ignition point of the polymer are the important factors that affect the combustion speed of pyrotechnic.The longer the side base of the binder,the higher the ignition point of the binder,the thicker the wall of the "net pocket",which slows down the heat and mass transfer between each "net pocket" combustion chamber,and makes the combustible and oxidant in each "net pocket" unit fully burn to form the overall slow combustion of pyrotechnics,thus enhancing the environment-friendly performance.The gas producing agent was prepared by using biological polyester as binder.The results of gas producing test show that the pyrotechnic with biological polyester binder can inflate the flexible air bag smoothly and slowly.The fireworks ignition test showed that the luminescent degree and color purity of the agent were increased.It provides innovative ideas for the research and development of new environment-friendly pyrotechnic binder. |
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